The present invention pertains to variable gain coder-decoders, and more particularly to a variable gain coder-decoder for use with telephone line signals.
Coder-decoders are used in telecommunications equipment for diverse purposes. Typically, a coder-decoder refers to an analog to digital converter and a digital to analog converter that are contained within the same circuit, such that two-way data communications may be performed between analog systems and digital systems. The coder-decoder thus xe2x80x9ccodesxe2x80x9d analog information into a digital format, and xe2x80x9cdecodesxe2x80x9d digital information into an analog format.
Coder-decoders may be used in conjunction with amplification when the signal received at the coder-decoder input would otherwise be too weak to be correctly decoded. External amplification devices may be utilized for such purposes, but these devices typically drive up the cost of the overall system that utilizes the amplifier and the coder decoder. They must be separately manufactured and installed, which creates additional costs and decreases overall system reliability. For this reason, coder-decoders with internal amplification systems have been developed.
Nevertheless, such coder-decoders with internal amplification systems have drawbacks that limit the usefulness of the coder-decoder. If a large amplification gain is used, the amplification system may be easily driven into saturation. If a small amplification gain is used, then it may still be necessary to have additional external amplification installed. For these reasons, it is often necessary to design the amplification for each coder-decoder to meet the needs of the system that it is being used in.
For certain applications, this constraint for coder-decoders can result in non-optimal configurations. For example, it is often desirable to obtain signals from a telephone line, such as caller identification signals, while the line is on-hook. An AC bypass path may be provided by connecting a capacitor across the hook relay. Known coder-decoders require a large capacitance to be used, because otherwise, signal attenuation will result in a caller identification signal that is too weak to be used. If the signal is amplified, then the data channel signal (carrying voice data, digitally-encoded data, or other suitable communications data that may be transmitted when the receiver is off-hook) that is received when the line is off-hook will saturate the amplifier. This configuration is undesirable, as it results in a hook impedance that does not meet industry standards.
The present invention provides a variable gain coder-decoder, in which the amplification gain may be adjusted to prevent the amplifier from being driven into saturation.
In accordance with one aspect of the present invention, a variable gain coder-decoder is provided. The variable gain coder-decoder includes a variable gain amplifier in which the amplification gain may be adjusted in one-decibel steps. An analog to digital converter is connected to the variable gain amplifier, receives the amplified output of the amplifier, and performs an analog to digital conversion of the amplified output.
In accordance with another aspect of the present invention, a system for transferring data over a communications medium, such as a telephone line, having a variable impedance load, such as a hook and a hook state, is provided. The system includes a relay coupled to the telephone, such as a hook relay. The hook relay can selectively block and transmit a communications signal from the telephone line. A bandpass filter is connected in parallel with the hook relay. A variable-gain coder-decoder is connected to the bandpass filter and the hook relay. The variable gain coder-decoder amplifies a bandpass signal at a high level when the hook relay is blocking, to compensate for signal loss through the bandpass filter. The variable gain coder-decoder amplifies the communications signal at a lower level when the relay is transmitting, because the signal loss through the bandpass filter is no longer present.
The variable gain coder-decoder of the present invention provides many important technical advantages. One important technical advantage of the present invention is a variable gain coder-decoder that may be used to process signals having a variable input level, such that a weak signal may be processed until such time as a stronger signal is received. The gain of the variable gain coder-decoder may then be adjusted in one-decibel steps so as to prevent the amplifier of the variable gain coder-decoder from being driven into saturation.
Those skilled in the art will further appreciate the advantages and superior features of the invention together with other important aspects thereof on reading the detailed description which follows in conjunction with the drawings.